Flexible circuit reflow soldering machine

ABSTRACT

A process and machine are disclosed for reflowing solder plated continuous flexible circuit webs. During reflow in a vapor environment, the flexible web is maintained in a planar orientation to produce a relatively uniform distribution of solder. Virtually all of the heat transfer fluid used in producing the vapor is recovered and retained.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 812,230, filed July 1, 1977,now U.S. Pat. No. 4,115,601.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process and machine for effecting solderreflow operations and, in particular, to a process and machine forreflowing solder plated continuous flexible circuit webs with negligibleloss of heat transfer fluid.

2. Description of the Prior Art

Several methods have been disclosed in the prior art for effectingsolder reflow operations on printed circuits through the use of hotsaturated vapors. One such method is briefly described in an articleentitled "Solvent Vapor Solder Reflow" by E. G. Dingman, appearing inthe IBM Technical Disclosure Bulletin, Vol. 13, No. 3, August 1970, atpage 639. Dingman discloses use of boiling solvents to rapidly andselectively apply heat to small areas having high thermal conductivityto enable solder rework operations with materials and components thatare heat sensitive. It seems readily apparent that Dingman does notaddress the problems of handling large and continuous flexible circuitwebs or loss of the boiling solvent.

One method for continuously handling printed circuits is disclosed in R.C. Pfahl, Jr. et al U.S. Pat. No. 3,866,307, issued Feb. 18, 1975. Inthis method individual circuit boards are loaded onto a conveyor andpassed through a receptacle containing hot saturated vapors of anexpensive fluid and a wave soldering font. Individual circuit boards areheated by the vapors and skim the solder wave at a low point of theconveyor catenary. One problem resulting from the Pfahl, Jr. et alapproach is that solder tends to pool at the low point of the catenary.Another problem is that despite attempts to retain the expensive fluid,substantial quantities are dragged out of the receptacle along with theconveyor and the circuit boards themselves.

Another method disclosed in T. Y. Chu et al U.S. Pat. No. 3,904,102,issued Sept. 9, 1975, attempts to reduce loss of the expensive fluid byuse of a less expensive vapor blanket atop the primary vapor zone. Oneembodiment of the Chu et al method utilizes batch processing techniques.A group of printed circuits is lowered into a receptacle containing theprimary vapor zone and the secondary vapor blanket. In anotherembodiment a conveyor carries the individual circuits into the vaporzone. However, in both embodiments significant quantities of theexpensive primary fluid are still lost. Moreover, the second embodimentcontinues to suffer from solder pooling effects. The first embodimentobviously is not readily adaptable for handling continuous webs ofprinted circuits.

A somewhat related application of the use of hot vapors is disclosed inK. W. Kamena U.S. Pat. No. 3,737,499, issued June 5, 1973. The Kamenamethod is used for modifying plastic surfaces on articles ofmanufacture. An individual plastic article is inserted into amulticompartmented chamber containing one or more vapor regions. Theheated vapors impinge on the surfaces of the plastic articles anddissolve at least a molecular layer to remove any surface blemishes andproduce a smooth, continuous finish. Kamena, like Pfahl, Jr. et al andChu et al, also suffers loss of the vapor material through web dragout.

Accordingly, it is one object of the present invention to implementsolder reflow operations on a continuous, flexible circuit web withoutsolder pooling.

Another object is to virtually eliminate any distortion of the webdimensions during solder reflow operations.

Still another object of the present invention is to substantially reducethe possibility of dielectric deterioration caused by the solder reflowprocess.

Yet another object is to virtually eliminate solder slivers producedduring etching operations.

A further object of the present invention is to reveal anydiscontinuities in the printed circuit which may have been bridged bysolder during the solder plating process.

Still a further object of the present invention is to indicate thesolderability of the flexible circuits.

Yet another object is to facilitate visual inspection of the flexiblecircuits.

Still a further object is to improve the appearance of the flexiblecircuits.

An even further object is to significantly reduce, if not virtuallyeliminate, any loss of the expensive working fluid resulting from webdragout, diffusion and convection.

SUMMARY OF THE INVENTION

The foregoing and other objects of the invention are realized in anillustrative embodiment of a process and machine for processing areflowable solder-plated flexible circuit web wherein the flexiblecircuit web is introduced into a first chamber having a vapor diffusiontrap at an entry port and a liquid seal at an exit port. The temperatureof the flexible circuit web is controlled in the first chamber to apoint below the solder eutectic temperature. Moreover, provision isincluded in the first chamber for positioning the flexible circuit webin a planar orientation for entry into a second chamber, containing acondensing vapor, at a point below the vapor-air interface. Thiscondensing vapor is confined in the second chamber by the liquid seal.Upon entry into the second chamber the flexible circuit web is exposedto the condensing vapor for a time sufficient to melt and reflow thesolder while maintaining on the flexible circuit web a condensate filmto aid in subsequently evaporatively cooling the flexible circuit webbelow the solder eutectic temperature. Residual traces of condensate onthe flexible circuit web are recaptured and thereafter the flexiblecircuit web is removed through an exit port.

Accordingly, it is one feature of the present invention that theflexible circuit web undergoes a preheating operation during passagethrough the liquid seal.

Another feature is that the liquid seal confines vapors of the expensiveworking fluid internal to the machine and keeps the solder below theeutectic temperature prior to entry into the condensing vapors.

Yet another feature of the present invention is that the flexiblecircuit web is positioned at low web tension in a planar orientationduring passage through the vapor zone thereby virtually eliminatingsolder pooling on the web.

Another feature is that oxidizing environments are avoided during solderreflow operations.

Yet another feature of the present invention is that flux applicationand its subsequent removal are avoided.

A further feature is that a sufficient vapor condensate film is retainedon the flexible circuit web after exiting from the vapor zone tomaterially aid in evaporatively cooling the reflowed solder below itseutectic temperature before mechanically contacting surfaces which mightredistribute the reflowed solder.

Still a further feature of the present invention is that the liquid sealat the entry side of the vapor chamber and a plurality of reheat rollersand diffusion traps at the exit side of the vapor chamber prevent anysignificant loss of the expensive working fluid through web dragout.

An even further feature is that the plurality of reheat rollers anddiffusion traps facilitate recapture and reuse of any residual traces ofcondensate on the continuous, flexible circuit web.

Yet a further feature of the present invetion is that the machine can beadvantageously elevated on its output side between an angle of 10 to 30degrees to achieve a range of planar orientations of the web withrespect to horizontal, to facilitate return of the recaptured workingfluid to its reservoir, and to control the amount of solder slump.Specifically, the machine parameters, including slope, may be adjustedadvantageously to process a wide variety of flexible circuits over awide range of throughput speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned objects and features of the invention, as well asother objects and features, will be better understood upon considerationof the following detailed description and appended claims, taken inconjunction with the attached drawings of an illustrative embodiment inwhich:

FIG. 1 illustrates a flexible circuit web having a number ofelectrically conductive patterns thereon and plated-through holestherethrough;

FIG. 2A is a simplified embodiment illustrating the solder reflowprocess;

FIG. 2B defines the shading code used in FIG. 2A and all similarlyshaded FIGS.;

FIG. 3 is an alternate simplified embodiment illustrating the solderreflow process;

FIG. 4 is a simplified perspective view of a solder reflow machine usedin practicing the process, in particular, illustrating the slopeadjustability feature and the threading aid feature;

FIG. 5A is a sectional view along line 5A--5A of FIG. 4 illustrating thevarious chambers and rollers utilized in the solder reflow machine topractice the process;

FIG. 5B is a sectional view along line 5B--5B of FIG. 5A illustratingthe doghouse-like cooling arrangement;

FIG. 6A illustrates the main condenser adjustability, slope adjustment,and web tension control and drive system features;

FIGS. 6B and 6C are partial cutaway views further showing condenseradjustability;

FIG. 7 is a partial cutaway view illustrating wiper arrangements used atboth the entry and exit ports of the machine to further aid in recaptureof the working fluid; and

FIG. 8 is a silicon-controlled rectifier circuit for maintaining uniformheating temperatures and for minimizing the temperature of the heatingelements to thereby increase the lifetime of the working fluid.

DETAILED DESCRIPTION

A flexible circuit web 100, as shown in FIG. 1, is comprised of adielectric substrate 101 onto which is bonded a patterned conductivefoil 102. The patterned conductive foil 102 is utilized to effectelectrical circuit connections among a plurality of electric circuitcomponents (not shown). Conductive foil 102 can be advantageously bondedon one or both sides of flexible circuit web 100. On double-sidedcircuits, the patterns of conductive foil 102 are generallyinterconnected, for example, by one or more plated-through holes 103.

In manufacturing flexible printed circuits, a solder coating 104 isplaced atop conductive foil 102 for several reasons. First, in numerousapplications, solder coating 104 is used as an etch resist. In the etchresist application of solder coating 104, oftentimes underetching occursnear the edges of conductive foil 102. This leaves a solder lipprojecting outwardly from conductive foil 102. Such lips are susceptibleto fracture and the formation of slivers during handling or subsequentprocessing. These slivers of solder can cause shorts between electricalcircuits, thereby causing circuit failure. Second, solder coating 104inhibits oxidation and corrosion of conductive foil 102 to reduce thepossibility of circuit failure through these mechanisms. Third, soldercoating 104 enhances solder wetting of the circuit during subsequentsolder assembly operations.

Implementation of reflow soldering, by virtue of the surface tensioncharacteristics of solder, causes these solder lips, when molten, todraw up onto conductive foil 102. A further advantage of reflowsoldering is that on double-sided circuits, solder sometimes bridgesgaps in conductive foil 102. These solder bridges may disguise defectsin the circuit which might lead to subsequent failure. Solder refloweliminates these bridges and exposes possible circuit defects.

Solder reflow also provides a means for perceiving solder wetability,and consequently provides a measure of acceptability for furtherprocessing. An additional advantage of solder reflow is that it aids inimproving the cosmetic appearance of the circuit, thereby enhancingcustomer acceptability.

Illustrated in FIG. 2A is a simplified embodiment of a solder reflowmachine 110. This simplified version facilitates an understanding of thedetails of the solder reflow process. The solder reflow machine 110 iscomprised of an enclosure 120 having a top 121, a bottom 122, and a pairof sidewalls 123 and 124. The remaining two sidewalls are not shown inorder to facilitate this description. A baffle 125 extends upwardly froman intermediate point of bottom 122 to a point spaced apart from top121. Another baffle 126 extends downwardly from an intermediate point oftop 121 to a point spaced apart from bottom 122. Baffles 125 and 126separate enclosure 120 into four definable compartments. Thesecompartments are hereinafter referred to, in the course of thisconceptual description, as first and second sumps, 127 and 128, andfirst and second chambers, 130 and 131. Chamber 130 spans sump 127 and aportion of sump 128. Chamber 131 spans the remainder of sump 128. Eachof chambers 130 and 131 has a port 132 and 133, respectively, associatedtherewith near top 121. Set apart from bottom 122 in sump 128 ispositioning roller 129.

A single noncorrosive working fluid 140 having a boiling point atatmospheric pressure sufficiently in excess of the liquidus temperatureof the solder to be reflowed is contained in sumps 127 and 128 which, asnoted previously, are separated from one another by baffle 125. Baffle125 is of sufficient height to keep the working fluid 140 contained insumps 127 and 128 generally separated. However, baffle 125 is not sohigh that a portion of working fluid 140 in sump 128 cannot spill overinto sump 127. Heating elements 141 located in sump 127 boil workingfluid 140 to produce a vapor more dense than air. The resulting vaporforms a vapor zone 142 which partially fills chamber 130. The height ofvapor zone 142 is controlled in chamber 130 by a plurality of condenserelements 143 and 144 along sidewall 123 and baffle 126, respectively.

As illustrated in FIG. 2A, flexible circuit web 100 passes over feedroller 150, enters port 133 in chamber 131, passes into working fluid140, and is routed about roller 129. The temperature of working fluid140 in sump 128 is maintained below the solder eutectic temperature ofsolder coating 104 on web 100 by temperature control element 138.However, the temperature is high enough to perform some preheating ofweb 100.

After passage around roller 129, web 100 with solder coating 104 thereonpasses into vapor zone 142. The heated vapors in vapor zone 142 condenseonto the relatively cool web, thereby effectively heating web 100 to atemperature above the solder liquidus temperature, melting soldercoating 104 and causing it to reflow. During reflow, web 100 is moved ina planar orientation at a selectable angle with respect to a horizontalplane. This orientation ensures that solder coating 104, after reflow,is maintained at a generally uniform thickness while in its moltenstate.

Following passage through vapor zone 142, flexible circuit web 100passes through cooling element 139, diffusion trap 134, and out throughport 132 in first chamber 130 where it passes over a cooled dischargeroller 151 to a take-up reel (not shown). The cooling of dischargeroller 151 is accomplished in a well-known manner and, hence, thedetails of such cooling are not specifically illustrated. To preventsolder smearing, some form of cooling is desirable prior to web 100being brought into contact with discharge roller 151. In thisembodiment, after passage through vapor zone 142 sufficient quantitiesof vapor condensate are retained on web 100 such that during theremainder of the time web 100 is contained within chamber 130 thecondensate film evaporatively cools the reflowed solder below itseutectic temperature.

Escape of any vapors of working fluid 140 through ports 132 and 133 isessentially prevented by vapor diffusion traps 134 and 135 positionednear ports 132 and 133, respectively. Diffusion traps 134 and 135 areshown as simplified structures in FIGS. 2A and 3 so as not toovercomplicate the description at this point. A more specific structureof diffusion traps 134 and 135 will be described in reference to FIG.5A.

In the alternate embodiment shown in FIG. 3, flexible circuit web 100 isfed over roller 150 through port 132 in sidewall 123 and throughdiffusion trap 134 into chamber 130. Upon entry of web 100 into chamber130, it is exposed to vapor zone 142. The hot vapors melt solder coating104 on web 100, causing the solder to reflow. Following passage throughvapor zone 142, web 100 passes through liquid seal 137 at exit port 136separating chamber 130 from sump 128. Once in sump 128, web 100 isfurther cooled by passage through cooling elements 145.

To insure that web 100 is maintained in a planar orientation during itspassage through vapor zone 142, positioning roller 129 in sump 128 isused in conjunction with roller 150 to control the orientation of web100 during this phase of the process. It should be noted that in theembodiment illustrated in FIG. 2A similar positioning effects areachieved with comparable roller 151.

Additional cooling is provided following passage of web 100 aroundroller 129, so that upon emerging from sump 128, solder coating 104 onweb 100 is well below its euctetic temperature. At this point, web 100is withdrawn from chamber 131 through diffusion trap 135 and out throughport 133 where it passes over roller 151 and is taken up by a take-upreel (not shown).

Regardless of which embodiment is used, these embodiments being shown inFIGS. 2A and 3, one significant feature is that during passage of web100 through vapor zone 142, web 100 is maintained in a planarorientation. This orientation insures a relatively uniform thickness ofthe layer of solder coating 104 on web 100 following the reflow process.Another feature is that the use of liquid seal 137 between sumps 127 and128 significantly aids in preventing the loss of vapors of working fluid140. Diffusion traps 134 and 135 at entry and exit ports 132 and 133further aid in reducing the amount of loss of working fluid 140.

The preferred embodiment for machine 110 used in implementing the solderreflow process is shown in FIG. 4 in outline form. Specificallyillustrated is apparatus on machine 110 for aiding in the threading offlexible circuit web 100 through the various chambers in the machine.Also specifically shown is apparatus which facilitates elevation of oneend of machine 110 with respect to an opposite end thereof whichapparatus controls the angle between a plane containing web 100 and ahorizontal plane.

To aid in threading flexible circuit web 100 through the variouschambers in machine 110, there are affixed to sidewall 123 a pair ofpulleys 154 and 155. Idler pulleys 156 and 157 at the end of roller 150and pulleys 160 and 161 at the end of roller 151, are affixed to top121. Additional idler pulleys 158 and 159 are affixed to sidewall 124.Similar idler pulleys (not shown) are affixed to the ends of rollers129, 167, 168, and 169 internal to machine 110.

Looped around pulleys 154, 156, 158, and 160 is a continuous, flexibletransport member 162, and looped around pulleys 155, 157, 159, and 161is a similar transport member 163. Transport members 162 and 163 may beadvantageously, for example, continuous cables, chains or the like.

In order to drive transport members 162 and 163 there is coupled topulleys 154 and 155 a shaft 149 which has affixed thereon, at anintermediate point along its length, a drive pulley 147. Motor 146,mounted on sidewall 123, is coupled to drive pulley 147 by drive belt148. When motor 146 is actuated, shaft 149 rotates, and this rotationforces transport members 162 and 163 to threadably traverse the variouschambers in machine 110.

Coupled to transport members 162 and 163 is bar 165. Bar 165, whenfastened to transport members 162 and 163, enables flexible circuit web100 to be looped therearound and fastened onto itself. Upon actuation ofmotor 146, web 100 is carried via transport members 162 and 163 and bar165 through machine 110. Once flexible circuit web 100 is threadablyinserted through the various chambers in machine 110, motor 146 isstopped and bar 165 can be removed from transport members 162 and 163.Thereafter, flexible circuit web 100 can be brought into engagement witha take-up reel (not shown). Once web 100 is threadably inserted intomachine 110, the solder reflow process becomes continuous merely byfastening one flexible circuit web 100 to another by means such asstapling the two webs together.

To facilitate elevation of one end of machine 110, for a purpose tobecome apparent subsequently, machine 110 has bottom edge 192 affixed toframe 170 by pivot 171. Elevation strut 172, pivotally mounted toopposite bottom edge 191 of machine 110, permits raising edge 191relative to edge 192. Adjustment of the slope between an angle of 10 to30 degrees is readily implemented by changing the attachment position ofstrut 172 to frame 170 along a plurality of apertures 173. Once theappropriate elevation is selected, strut 172 is held juxtaposed theappropriate aperture 173 by a holding pin (not shown).

The preferred embodiment of machine 110 is shown in cross sectional viewin FIG. 5A. Flexible circuit web 100 enters machine 110 by downwardlydeflecting wiper assembly 177 at entry port 133. Wiper assembly 177,similar to that shown in FIG. 7, along with baffles 180 and 181 and theclose spacing along diffusion trap 135, virtually prevent escape of anyworking fluid 140 from machine 110 at the point of entry of flexiblecircuit web 100. Following passage through diffusion trap 135, web 100is immersed into fluid 140 in sump 128 and is passed around roller 129.Roller 129 positions web 100 for entry into chamber 130 so that web 100will encounter vapor zone 142 in a planar orientation below the vaporair interface. Moreover, temperature control element 138 controls thetemperature of fluid 140 so that it preheats solder coating 104 on web100 to just below the solder eutectic temperature. In the event thetemperature of fluid 140 is sufficiently high so as to generate anyvapors, these vapors, upon exposure to diffusion trap 135, are condensedinto liquid form. Consequently, troughs 195 are provided at lower endsof diffusion trap 135 so that any recondensed vapors are returned tosump 128 rather than forming on web 100.

After passing around roller 129, web 100 is routed through passageway136 separating sump 128 from vapor zone 142. Passageway 136 is formed bybaffle 126 which extends from top 121 downwardly to a point spaced apartfrom bottom 122. To preclude escape of vapors from chamber 130, thefluid in sump 128 is maintained at a level just above passageway 136.Hence, web 100 enters chamber 130 through liquid seal 137.

Vapor zone 142 in chamber 130 is generated by boiling fluid 140 in sump127 by heating elements 141. The temperature of heating elements 141 iscontrolled in order to maintain fluid 140 in contact with them at anearly uniform temperature. This is effected by making heating elements141 such that they have a uniform resistance per unit of length. Thisuniform resistance ensures that a proportionate reduction in power willproduce a proportionate reduction in heat flux. The reduction in heatflux results in fewer hot spots being formed in fluid 140 and this, inturn, improves the usable lifetime of fluid 140.

Control of the height of the vapor air interface is effected bycondenser elements 143 and 144 positioned along enclosing surfaces ofchamber 130 containing vapor zone 142. The location of condenserelements 143 and 144, which elements are adjustable as shown mostclearly in FIGS. 6B and 6C, along with the elevation angle of bottomedge 191 of machine 110 and the speed of travel of web 100 fixes thetime of exposure of solder coating 104 on web 100 to vapor zone 142.

Baffles 182 and 183 are positioned just below the vapor air interface todecrease the amount of convection air interacting with solder coating104 during its passage through vapor zone 142. This results in a moreuniform exposure of web 100 to the hot vapors of vapor zone 142 and thisin turn results in an improved cosmetic appearance of flexible circuitweb 100 because the presence of air in the vicinity of the reflowedsolder tends to oxidize the solder thereby dulling the finish. In orderto prevent any droplets of fluid 140 from entering vapor zone 142, ademister unit 197 is provided in sump 127.

During the passage of flexible circuit web 100 through vapor zone 142,the hot vapors heat solder coating 104 above its liquidus temperaturecausing the solder to reflow. Since web 100 is maintained in a planarorientation during this passage the effects of solder slumping arereduced and solder coating 104 is provided with a more uniformdistribution. Moreover, by controlling the transit time and planar angleof web 100 as it passes through vapor zone 142, sufficient condensate isallowed to form on web 100.

The formation of this condensate materially aids in the cooling of web100 as it passes between upper condenser element 184 and lower condenserelement 185. Cooling at this point is desirable in order to bring thetemperature of solder coating 104 below its eutectic temperature priorto its being brought into contact with roller 167.

Upper condenser element 184, as shown in FIG. 5B, has a doghouse-likeshape so that any condensate driven off web 100 onto this condenser isprevented from dripping back onto web 100. To achieve this effect uppercondenser element 184 includes oppositely directed members 201 and 202,as shown in FIG. 5B, which members are oriented at a common angle withrespect to a plane containing flexible circuit web 100.

As a first step in prohibiting escape of vapors of working fluid 140,baffle 203 separates chamber 130 from the follow-on stages used torecapture any residual traces of fluid 140 which may have a tendency toescape through web dragout. Following cooling by virtue ofdoghouse-shaped condenser elements 184 and 185, web 100 engages aplurality of reheat rollers 167 through 169 and a correspondingplurality of diffusion traps 186, 190, and 134, respectively. Roller 167and diffusion trap 186 are separated from roller 168 and diffusion trap190 by baffles 187 and 188. Similarly, roller 168 and diffusion trap 190are separated from roller 169 and diffusion trap 134 by baffles 188 and189.

Upon engagement of web 100 with each of rollers 167 through 169, it isreheated to a temperature just below the eutectic temperature of soldercoating 104. This reheating vaporizes any residual traces of condensateof fluid 140 so that upon entering diffusion traps 186, 190, and 134,this condensate is removed from web 100. The presence of baffles 187through 189 ensures that with each successive stage lesser amounts ofcondensate are available for removal from machine 110 by web dragout.

To facilitate return of the recaptured condensate to sump 127 each ofdiffusion traps 186, 190 and 134 are provided with troughs 195 at theirlower extremities. Troughs 195 reduce the possibility of recapturedcondensate coming into contact with web 100.

Following passage of web 100 around reheat roller 169 and throughdiffusion trap 134, it emerges through exit port 132. To further inhibitescape of any vapor of fluid 140, diffusion trap 134 is coupled to exitport 132 by baffles 205 and 206 which are spaced closely together. Inaddition, exit port 132 is equipped with a wiper assembly 178, as shownin FIG. 7, which further provides for removal of any traces of vapor offluid 140 carried by web 100.

It should be noted that during passage of web 100 through vapor zone142, web 100 is maintained in a planar orientation even at low webtensions. This effect is achieved with rollers 129, 167, 168, and 169along with input roller 150 and discharge roller 151. The manner inwhich this effect is achieved will become clear upon consideration ofFIG. 6A. Moreover, as noted above, condenser elements 143 and 144 areadjustable so that along with the speed of travel of web 100 and theangle of elevation of bottom edge 191 with respect to bottom edge 192 ofmachine 110, the exposure time of solder coating 104 to vapor zone 142can be accurately controlled. The manner of adjustment of condenserelements 143 and 144 will be considered subsequently.

Illustrated in FIG. 6A is apparatus for transporting flexible circuitweb 100 through machine 110 such that web 100 is maintained in a planarorientation at low web tension during its passage through vapor zone142. In particular, motor 220 mounted on top 121, shown only in FIG. 6Afor clarity, drives discharge roller 151 and intermediate rollers 167through 169 by drive chains 221. To control the tension in web 100 as itis fed from supply roller 250, variably adjustable tension roller 251 isused. Tension roller 251 is coupled to pneumatic constant load device253 by tension control arm 252. Coupled to tension control arm 252 istension arm position sensor 254.

If the feed rate of web 100 into machine 110 slows down relative to themachine output speed, the amount of web 100 looped around tension roller251 decreases and tension control arm 252 swings in an upward direction.Simultaneously, tension arm position sensor 254 detects this change inposition of tension control arm 252 and as a result an electrical signalis produced which causes drive motor 220 to slow down. The decrease inrotational speed of drive motor 220 arrests the imbalance between thefeed rate and machine output speed, thereby effectively controlling theamount of web looped around tension roller 251 so that the tension inweb 100 is held nearly constant during its passage through machine 110.

To further insure that web 100 passes through vapor zone 142 in a planarorientation, bottom edge 191 of machine 110 is adjustable with respectto bottom edge 192. This elevation adjustability aids in providing amore uniform thickness to solder coating 104 following the solder reflowprocess without the detrimental effects caused by solder slump inherentin catenary feed arrangements. This elevation adjustability further aidsin controlling the amount of vapor condensate remaining on web 100 as itrises above the vapor air interface for evaporative cooling.

Control of the height of vapor zone 142 is achieved by the adjustabilityof condenser elements 143 and 144. Each of condenser elements 143 and144, as shown most clearly in FIGS. 6B and 6C, respectively, includesmovable pans 230 and 230' for housing the condenser elements themselves.Pans 230 and 230' can be moved advantageously in either a generallyvertical or generally horizontal direction, as appropriate. To effectthis movement pans 230 and 230' are coupled via shafts 231 and 231' tohand cranks 232 and 232', respectively.

Since condenser elements 143 and 144 provide direct cooling, it isdesirable that any condensate forming thereon be returned directly tosump 127 without coming into contact with flexible circuit web 100. Toachieve this end pans 230 and 230' are provided with telescoping tubes235 and 235' which couple pans 230 and 230' directly to sump 127regardless of the position of condenser elements 143 and 144 withrespect to sump 127. This arrangement has the further effect ofminimizing the production of excessively hot vapors needed to maintainvapor zone 142 and this, in turn, increases the useful lifetime ofworking fluid 140.

Additional measures used to prevent the loss of fluid 140 arerepresented by the wiper arrangement shown in FIG. 7 and brieflydiscussed earlier. Specifically illustrated in cross sectional form isthe wiper arrangement at exit port 132. Flexible membranes 260 and 261are affixed in overlapping alignment along edges 262 and 263,respectively, of exit port 132 by rigid members 264 and 265. As web 100emerges from exit port 132, membranes 260 and 261 are flexed outwardlyforming a seal about web 100. This seal prevents any convective loss ofvapor due to web 100 dragging out the mixture of air and vapor existingnear diffusion trap 134.

As noted previously, heating elements 141 are used to control theboiling rate of fluid 140 and to avoid the production of hot spotsthereby increasing the lifetime of fluid 140. Besides making heatingelements 141 such that they have a uniform resistance per unit oflength, the power to them is controlled so that a proportionatereduction in power produces a proportionate reduction in heat flux. Thisresult is achieved by the silicon controlled rectifier circuitillustrated in FIG. 8.

Three phase, 60 cycle AC commercial power is coupled through fuses 290to filter capacitors 291. Following each capacitor 291 there is aparallel circuit comprised of diode 292 and a silicon controlledrectifier 293. Diode 292 provides rectification of the AC power andsilicon controlled rectifier 293, by virtue of a trigger bias voltagecoupled thereto, fairly accurately controls the amount of rectifiedpower supplied to heating elements 141. This arrangement insures thatall heating elements 141 are activated uniformly which, in turn,virtually eliminates hot spots in fluid 140 thereby increasing itsuseful lifetime.

In all cases it is to be understood that the above-described embodimentsare but representative of many possible specific embodiments which canbe devised readily in accordance with the principles of the disclosedinvention. Thus, numerous and various other embodiments can be effectedreadily by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is:
 1. A machine for processing reflowable solder platedflexible circuit webs comprising:an enclosure containing first andsecond sumps and first and second chambers, said first chamber spanningsaid first sump and a portion of said second sump and said secondchamber spanning the remainder of said second sump, each of saidchambers having a port therein; a single noncorrosive fluid having aboiling point at atmospheric pressure sufficiently in excess of theliquidus temperature of the solder to be reflowed, said fluid in itsliquid state occupying said first and second sumps and in its vaporstate occupying a portion of said first chamber; means for boiling saidliquid in said first sump to generate said vapor occupying said portionof said first chamber; means for maintaining said liquid in said secondsump at a temperature such that said solder temperature is below itseutectic point; means for controlling said vapor at a predeterminedlevel above said first sump; means for moving said web through saidvapor occupying said portion of said first chamber, said vapor meltingsaid solder causing it to reflow, said web being moved along a planeoriented at a predetermined angle with respect to a horizontal planesuch that said solder is maintained at a generally uniform thicknesswhile molten; and means for cooling said solder after exiting from saidvapor.
 2. The machine in accordance with claim 1 wherein said first sumpvapor controlling means comprisesfirst and second condenser elementspositioned along enclosing surfaces of said chamber containing saidvapor.
 3. The machine in accordance with claim 1 wherein said boilingmeans comprises:a plurality of heating elements; and means forcontrolling the temperature of said heating elements to maintain saidfluid in contact therewith at a nearly uniform temperature.
 4. Themachine in accordance with claim 3 wherein said plurality of heatingelements have a uniform resistance per unit of length which allows aproportionate reduction in power to produce a proportionate reduction inheat flux thereby reducing the production of hot spots in said fluid andimproving the usable lifetime of said fluid.
 5. The machine inaccordance with claim 1 wherein said moving means comprises:a firstroller, at least a portion of which is immersed in said fluid in saidsecond sump, for guiding said flexible circuit webs therearound; atleast one other roller external to an exit port having said flexiblecircuit webs in contact therewith; means for driving said other roller;and means for controlling said driving means so that the tension in saidwebs is just sufficient to maintain them in a planar orientation duringpassage through said vapor.
 6. The machine in accordance with claim 5wherein said driving tension controlling means comprises:a tensionmaintaining roller having said flexible circuit webs in contacttherewith; a pneumatic constant load device; means, pivotally mounted onsaid enclosure and having one end coupled to said tension maintainingroller and an opposite end coupled to said pneumatic load device, formaintaining a force on said tension roller in proportion to the tensionexerted on said webs; and means, coupled to said maintaining means, forproviding an electrical signal to said driving means to adjust itsrotational speed.
 7. The machine in accordance with claim 1 furthercomprising:means, threadably traversing said enclosure, for facilitatingthreading of said flexible circuit webs therethrough; and means fordriving said facilitating means.
 8. The machine in accordance with claim7 wherein said facilitating means comprises:first and second pluralitiesof pulleys; first and second continuous, spaced-apart, flexibletransport members coupled around said pulleys; and a rigid memberconnected between said first and second transport members.
 9. Themachine in accordance with claim 8 wherein said driving meanscomprises:a motor; a shaft affixed to first and second opposed pulleysin said first and second pluralities of pulleys, respectively; a drivepulley affixed to said shaft at an intermediate point along its length;and means for coupling said motor to said drive pulley so that rotationof said motor causes said transport members to be threaded through saidenclosure from an entry port to an exit port.
 10. The machine inaccordance with claim 1 further comprising:a support frame; means forpivotally mounting a bottom edge of said enclosure to said supportframe; means for adjusting the elevation of an opposite bottom edge ofsaid enclosure with respect to said support frame; and means for lockingsaid adjusting means for the selected elevation.
 11. The machine inaccordance with claim 1 further comprising:means for decreasing the flowof convection air currents through said vapor; and means for removingdroplets of vapor which might pass from said fluid to said vapor. 12.The machine in accordance with claim 11 wherein said decreasing meanscomprises:a first baffle positioned slightly below an interfaceseparating said vapor from air; and a second baffle positioned justbelow said flexible circuit webs and extending between said first sumpand said vapor level controlling means.
 13. The machine in accordancewith claim 1 wherein said cooling means comprises a doghouse-shapedcondenser element having oppositely directed members positioned abovesaid flexible circuit webs said oppositely directed members oriented atcommon angles with respect to a plane containing said flexible circuitwebs so that condensate collected thereon is prevented from droppingonto said flexible circuit webs.
 14. The machine in accordance withclaim 1 further comprising:means for recapturing residual traces ofcondensate on said flexible circuit webs; and means for inhibitingescape of said vapor from said machine.
 15. The machine in accordancewith claim 14 wherein said recapturing means comprises:a plurality ofrollers for reheating said solder to a temperature below its eutectictemperature; a plurality of diffusion traps, one diffusion trapfollowing each of said rollers, said diffusion traps condensing vaporsdriven off said flexible circuit webs by said reheat rollers; means forpreventing condensed vapors from coming into contact with said flexiblecircuit webs; and means for separating associated reheat rollers anddiffusion traps from one another.
 16. The machine in accordance withclaim 14 wherein said inhibiting means comprises:first and secondoverlapping, opposed flexible membranes; and means for fastening saidmembranes along edges of an entry port and an exit port.
 17. The machinein accordance with claim 1 wherein said vapor level controlling meanscomprises:means for cooling air above said vapor; and means foradjusting the location of said cooling means.
 18. The machine inaccordance with claim 17 wherein said air cooling means comprises:firstand second condenser elements; and means for slidably mounting saidcondenser elements.